A multitude of sensor elements and methods for detecting at least one property of a measuring gas in a measuring gas chamber are known from the related art. Fundamentally, this can concern any physical and/or chemical properties of the measuring gas, it being possible to detect one or multiple properties. The present invention is described below in particular with reference to a qualitative and/or quantitative detection of a proportion of a gas component of the measuring gas, in particular with reference to a detection of an oxygen proportion in the measuring gas portion. The oxygen proportion can be detected for example in the form of a partial pressure and/or in the form of a percentage. Alternatively or additionally, however, other properties of the measuring gas are detectable as well such as the temperature, for example.
Such sensor elements can be designed as so-called lambda probes, for example, as they are known for example from Konrad Reif (Ed.): Sensoren im Kraftfahrzeug (Sensors in the Motor Vehicle), 1st Edition 2010, p. 160-165. Using broadband lambda probes, in particular planar broadband lambda probes, it is possible for example to determine the oxygen concentration in the exhaust gas within a great range and thus infer the air-fuel ratio in the combustion chamber. The air ratio λ describes this air-fuel ratio.
Ceramic sensor elements are known in particular from the related art, which are based on the use of electrolytic properties of certain solids, that is, on ion-conducting properties of these solids.
These solids can be in particular ceramic solid electrolytes such as zirconium dioxide (ZrO2) for example, in particular yttrium-stabilized zirconium dioxide (YSZ) and scandium-doped zirconium dioxide (ScSZ), which can contain small additions of aluminum oxide (Al2O3) and/or silicon oxide (SiO2).
Despite the advantages of the sensor elements known from the related art, these still leave room for improvement. Thus ceramic exhaust-gas sensors are used for measuring the concentration of oxygen and/or nitrogens in the exhaust gases of automobiles. Following the start of the engine, the ceramic sensors are heated by integrated heaters within a few seconds to an operating temperature of about 700° C. to 800° C. The time until the operating temperature is reached, the so-called fast light-off time, depends greatly on the heating power produced by the heater. The fast light-off time is also reduced the more the heating energy is introduced locally in proximity of the Nernst electrodes since the temperature is determined here by internal resistance measurement. The maximum heating power that can be introduced in the heater is limited inter alia by the maximum amperage of the output stage in the engine control unit, the maximally admissible temperature in the heater meander without damaging the material and the maximally occurring thermomechanical stresses due to temperature differences within the ceramics without the formation and growth of fissures. The function of such a sensor element necessitates an inner electrode cavity. This cavity represents a heat barrier, which inhibits the heat conduction between the heating element and the outer pump electrode. The cavity side facing the heating element heats up more quickly during the heating process than the side facing away from the heating element. This gives rise to thermomechanical stresses especially on the outer edge of the cavity, which are additionally increased by the notch effect of the cavity edge.